Contamination control, including particulate monitoring, plays a critical role in the manufacturing processes of several industries. These industries require clean rooms or clean zones with active air filtration and require the supply of clean raw materials such as process gases, deionized water, chemicals, and substrates. In the pharmaceutical industry, the Food and Drug Administration requires particulate monitoring because of the correlation between detected particles in an aseptic environment and viable particles that contaminate the product produced. Semiconductor fabrication companies require particulate monitoring as an active part of quality control. As integrated circuits become more compact, line widths decrease and wafer sizes increase such that the sizes of particulates causing quality problems become smaller. Accordingly, it is important to detect and accurately size submicron particles of ever decreasing sizes and numbers per volumetric unit.
To perform particulate monitoring, currently commercially available submicron particle sensors use optical detection techniques to determine the presence, size, and number of particles in a volumetric unit. The basic building block for this technology is intracavity optical scattering of a laser beam and detection of the optical signal scattered by the particles. The standard particle detection approach, which was developed during the late 1980s, passes a sample stream containing the particles through an elongated flattened nozzle such that the sample stream exiting the nozzle intersects the laser beam in an area referred to as a view volume. Scattered light from particles in the view volume is collected with optics and focused onto the detection system.
U.S. Pat. No. 5,642,193 for PARTICLE COUNTER EMPLOYING A SOLID-STATE LASER WITH AN INTRACAVITY VIEW VOLUME, which is assigned to the assignee of this application, describes such a particle detection and counting system along with techniques for improving particle sizing resolution. U.S. Pat. No. 4,746,215 for PARTICLE COUNTER AIR INLET ASSEMBLY describes a nozzle that produces a particle flow sample stream for developing a view volume in particle counting systems.
The nozzle, laser beam, and resulting view volume all have properties that affect particle detection sensitivity and sizing resolution. For example, the type of laser employed affects the laser beam lateral (transverse to the beam longitudinal axis) intensity profile, with prior art multispatial mode HeNe lasers typically having a "top-hat" shaped lateral intensity profile. Such lasers are used to provide sufficient intracavity optical power to produce detectable amounts of scattered light. To complement the laser beam top-hat intensity profile, the nozzles produced a turbulent flow having a substantially square lateral velocity profile where it intersects the laser beam. This is beneficial because the resulting view volume contains a top-hat lateral laser intensity profile and a complementary lateral particle velocity profile that together produce an overall uniform lateral particle detection sensitivity, which improves particle sizing resolution. Unfortunately, the turbulent nozzle flow produces intracavity noise in the view volume that degrades the particle detection signal-to-noise ratio. Reducing the nozzle flow rate to produce laminar flow results in a parabolic velocity profile that reduces intracavity noise, but also degrades particle sizing resolution and overall particle detection rate.
In addition to the above-described problems, the nozzles produce a sample stream longitudinal velocity profile that causes mono-disperse (uniformly distributed particles of the same size) particles to have different velocities depending on their longitudinal passage position through the view volume. Moreover, the laser beam intensity is not longitudinally constant along the view volume because of beam divergence and because the light collection optics cannot perfectly reproduce on the detection system image intensities generated in the view volume. Therefore, particle detection systems that detect light scattered from particles to produce pulse amplitudes indicative of particle size win, unfortunately, produce nonuniform output pulse amplitudes depending on the longitudinal positions of particles in the view volume.
What is needed, therefore, is a particle detection system having high, uniform particle detection sensitivity and sizing resolution throughout an entire view volume and, in particular, one that meets or exceeds the requirements set forth in JAPANESE INDUSTRIAL STANDARD JIS B 9921-1989 for a "Light Scattering Automatic Particle Counter."